Pat wrote:

I'm installing an undersink 4.5 gal. water heater and need a good way
to connect the 3/4" pressure relief valve to a 1-1/4" drain line.
The only fittings I've found that come close to this is a
dishwasher connection for 1-1/2" pipe. This would require re-plumbing
the sink with pop-up stopper for 1-1/2" pipe.

Other possibilities would be to run it through a brick wall or just
drain it into a container.

Any ideas? Do I need an air gap?

Thanks
pb

You could ask the local inspector, but I don't think discharging
the superheated water/steam into the basin drain will fly.

I would rather see it dischg to the outside. Bring the pipe to
6" to 24" above ground level and aimed down.

Jim